1. Field of the Invention
The present invention relates to an A/D conversion apparatus. The present application claims priority of Japanese Priority Patent Application JP 2010-247672 filed in the Japan Patent Office on Nov. 4, 2010, the entire content of which is hereby incorporated by reference.
2. Description of Related Art
A conventional analog/digital (A/D) conversion circuit provided in a consumer device such as a digital camera compares analog signals input from various sensors with a reference voltage of the A/D conversion circuit, thereby converting voltage values of the input analog signals into binary digital data. Moreover, when a change in the voltage values of the analog signals input to the A/D conversion circuit is minute, an analog amplification circuit such as an operational amplifier is used in order to ensure a voltage range necessary for A/D conversion. With such a configuration, the analog signals are amplified using the analog amplification circuit and amplified voltage values are subject to the A/D conversion, so that the binary digital data is obtained.
For example, in JP-A-5-259907, technology in which an A/D conversion circuit is completely realized by a digital circuit is disclosed, for example. In a time A/D conversion circuit disclosed in JP-A-5-259907, a pulse phase difference coding circuit is completely realized by a digital circuit, wherein the pulse phase difference coding circuit includes a pulse circulating circuit, a counter, a latch circuit, a pulse selector, an encoder, and a subtraction circuit.
In the time A/D conversion circuit disclosed in JP-A-5-259907, a pulse circulating circuit includes a plurality of inverting circuits and an analog voltage signal is input as a power supply voltage of the inverting circuits. Furthermore, the number of circulations of a pulse signal circulating in the pulse circulating circuit with a predetermined A/D conversion time is counted, and the position of an edge of the pulse signal is detected, so that digital data converted into binary data can be obtained.
JP-A-5-259907 discloses a technique in which a frequency variable oscillation circuit controls an oscillation frequency to change a predetermined A/D conversion time. For example, when a propagation delay time of the inverting circuit is shortened due to a reduction of an environmental temperature and a value of the digital data is increased, the frequency variable oscillation circuit controls the oscillation frequency to be increased, thereby shortening the A/D conversion time